| In order to stop particulate pollution from entering the human body and compromising health,air filtration products like masks and air purifiers are frequently utilized.This is due to the COVID-19’s slow spread and the worsening of the atmospheric environment.Using electret melt-blown nonwovens as their main ingredient,the vast majority of air filtration solutions on the market are able to achieve high filtration efficiency thanks to their advantages of extremely fine fibers,small pore size,and high porosity.However,electret melt-blown nonwovens are usually prepared from polypropylene,which is difficult to degrade,and large-scale disposal has caused a serious burden on the environment.Therefore,the development of environmentally friendly degradable electret melt-blown nonwovens is the current emphasis of air filtering research.Lactic acid produced by natural plant fermentation is used to chemically create polylactic acid（PLA）,a polyester polymer.It has good mechanical qualities,is processable,and is biodegradable and biocompatible.A recent hot topic in application research for PLA melt-blown nonwovens is in the area of air filtration.Electret technology refers to electrifying the air filter material by means of an external electric field or friction,thereby endowing it with an electrostatic effect.This can significantly increase its filtration efficiency without raising the pressure drop.As consumer demand for air filtering products has increased,traditional corona electret technology is difficult to meet the requirements of high filtration efficiency and long-term storage.The advantages of high filtration efficiency,low filtration resistance,and stable storage have made hydro-charging technology a hot topic.However,the hydro-charging production line is long,occupies a large area,and parameter adjustment is difficult.At present,there are few researches on hydro-charging technology,and the influence law of electret parameters and electret mechanism are not yet clear,which limits its application.In order to conduct in-depth research on the hydro-charging process parameters and electret mechanism,a small laboratory hydro-charging device was developed in this paper to realize the hydro-charging treatment and process parameters of PLA melt-blown nonwovens.In this project,a variety of PLA electret melt-blown nonwovens were created utilizing the melt-blown method and hydro-charging technology,and it was discovered how the material’s structure affected the electret effect.The effect of various factors on the composition and functionality of PLA melt-blown nonwovens was studied,along with the storage needs for materials used in air filters in practical uses,the electret properties of the materials in various temperature and humidity environments,and finally the charging and charge storage mechanisms of the hydro-charging PLA melt-blown nonwovens were examined.The main research results are as follows:（1）During hydro-charging treatment,the finer the fibers in the material,the more sufficient the contact and friction between the high-speed pure water jet and the fibers in the material,and the better the electret effect.When the frequency of the metering pump is 6 Hz,the average diameter of the fibers in the material is only 2.65μm,and the average pore size is15μm.After the hydro-charging,the filtration efficiency increases from 52.37%to 92.3%,and the pressure drop decreases from 13.37 MPa to 13.04 MPa,the average surface potential can reach 714 V.（2）By modifying the hydro-charging process settings,several PLA melt-blown electret nonwovens were created,and their shape,structure,and electret effect were evaluated and characterized.The surface morphology of the material did not significantly change after picking the best electret parameters of 2 MPa electret water pressure,2 m/min walking speed,and 40°C drying temperature,although the average pore size increased somewhat and the porosity fell from 82.7%to 90.76%.The filtration efficiency increased by more than 40%,the pressure drop decreased,the air permeability was improved,the quality factor of the material could reach 0.249 Pa-1,the average surface potential was 755 V,and the distribution uniformity was improved.（3）The effect of temperature treatment on the material’s capacity for charge storage is influenced by its characteristic temperature:when a material is processed at a temperature below the thermal deformation temperature,the charge loss of the material is mainly the surface unstable charge and some shallow well charges.As the processing temperature increases,the order of the crystal grains in the material is enhanced,and the deep wells at the interface increase.Part of the charges in the shallow wells dissipate after gaining energy,and the rest of the charges are captured by the increased deep wells.The effect of treatment humidity on the material’s ability to store charges is correlated with the conductivity of the material:the higher the treatment humidity,the more water molecules from the surrounding environment enter the material.When the treatment temperature is too high,the charges in the material gain higher energy and dissipate faster.Within,it takes on the role of a charge carrier,the material’s conductivity is improved,and the rate of charge loss is sped up.（4）Based on the experimental data and theoretical analysis,the mechanism of hydro-charging charge generation and charge storage is proposed.A single fiber experiences an electron transfer when the pure water jet comes into touch with it.As a result,the fiber sheds its electrons and gains a positive charge.A portion of the charge on the fiber’s surface migrates to the fiber’s interior during the hot-air drying process to create a stable charged electret material.During the process of generating charges and entering the inside of the fiber,low-energy charges are captured by shallow wells to become shallow well charges,and high-energy charges enter deep wells to become deep well charges.The greater the proportion of deep well charges,the more stable the storage. |
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